The
hydrolysis of -lactam antibiotics by -lactamases
is one of the major bacterial defense systems. These enzymes generally
hydrolyze a variety of antibiotics including the latest generation of
cephalosporins, cephamycins and imipenem. In this paper, the effect of
cephalosporins-based antibiotics on the peroxynitrite-mediated nitration of
protein tyrosine is described. Although some of the antibiotics have weak
inhibitory effect on the nitration reactions in the absence of
-lactamase, they exhibit very strong
inhibition in the presence of -lactamase. This is due to the
elimination of heterocyclic thiol/thione moieties from cephalosporins by -lactamase-mediated
hydrolysis. After the elimination, the thiols/thiones effectively scavenge
peroxynitrite, leading to the inhibition of the nitration reactions.

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